The words “staging” and “staging system” have a special meaning when it comes to cancer. According to the NCI Dictionary of Cancer Terms, staging means:
- “Performing exams and tests to learn the extent of the cancer within the body, especially whether the disease has spread from where it first formed to other parts of the body. It is important to know the stage of the disease in order to plan the best treatment.”
Similarly, a “cancer staging system” means:
- “A system that is used to describe the extent of cancer in the body. Staging is usually based on the size of the tumor and whether the cancer has spread from where it started to nearby areas, lymph nodes, or other parts of the body.”
Staging is of vital importance in making treatment decisions about any particular cancer. As the American Cancer Society website puts it:
- “For most types of cancer, doctors need to know how much cancer there is and where it is (among other things) to help determine the best treatment options. For example, the best treatment for early-stage cancer may be surgery or radiation, while a more advanced-stage cancer may need treatments that reach all parts of the body, such as chemotherapy, targeted drug therapy, or immunotherapy.”
Staging is thus an attempt to discover the true extent of cancer in the human body. To do so, physicians first look at the primary, or main, tumor. What are its size and location and has it grown into other nearby areas? They look for affected lymph nodes. Finally, they look to see if cancer has spread to other organs, including ones distant from the original primary site.
The stage of a cancer is typically a composite of these three figures. For a long time, physicians relied upon various schemes, such as the so-called Jewett-Strong-Marshall system for bladder cancer, which measured the depth of invasion of the tumor. But today almost everyone uses the universal TNM system. This gives a more nuanced picture of the local, regional, and general status of the disease.
The TNM Cancer Staging System
The TNM system is an almost universally adopted method of classifying all solid tumors by reference to three key characteristics. These are the size and extent of the tumors (the “T” in TNM), the spread of cancer to the lymph nodes (the “N”), and the extent of non-local metastases (the “M”). Two organizations, the American Joint Committee on Cancer (AJCC) and the Union for International Cancer Control (UICC), maintain the TNM classification system worldwide to help doctors stage many different types of cancer, based on common standards.
In the TNM system, the stage is determined AFTER cancer has been assigned a letter or number to describe the key features of this particular disease.The TNM score briefly summarizes crucial information that helps assign this particular cancer to its current stage. This is a process of determining the degree to which any particular cancer has grown or spread. Typically, doctors assign a Roman numeral from I to IV to any solid cancer. The letter “I” refers to an isolated tumor, while IV is cancer that has spread distantly (or is at its greatest extent).
Here is a more detailed description of the various TNM categories. We have borrowed it from the ACS website:
The Primary Tumor (T category)
“The T category can be assigned a letter or a number:
- TX means there’s no information about the primary tumor, or it can’t be measured.
- T0 means there is no evidence of a primary tumor (it cannot be found).
- Tis means that the cancer cells are only growing in the layer of cells where they started, without growing into deeper layers. This may also be called in situ cancer or pre-cancer.
- A number after the T (such as T1, T2, T3, or T4) might describe the tumor size and/or amount of spread into nearby structures. The higher the T number, the larger the tumor and/or the more it has grown into nearby tissues.
The Lymph Nodes (N category)
Lymph nodes near the primary tumor are usually checked to find out if cancer has spread into them. Lymph nodes are small, bean-shaped collections of immune cells. Many types of cancer often spread to nearby lymph nodes before they reach other parts of the body.
The N category can be assigned a letter or a number:
- NX means there’s no information about the nearby lymph nodes, or they can’t be assessed.
- N0 means nearby lymph nodes do not contain cancer.
- A number after the N (such as N1, N2, or N3) might describe the size, location, and/or the number of nearby lymph nodes affected by cancer. The higher the N number, the more cancer has spread to nearby lymph nodes.
Cancer Metastasis (M category)
Doctors might also look at other parts of the body to see if cancer has spread. Cancer that has spread to parts of the body far from the primary tumor is known as metastasis and contributes to the staging equation.
The M category is assigned a number:
- M0 means that no distant cancer spread has been found.
- M1 means that cancer has been found to have spread to distant organs or tissues.
[End of our borrowing from the ACS Website]
Typically, doctors use the TNM system when talking among themselves. The end result of this is to assign a stage to cancer. Patients are usually only made aware of the stage of their cancer, not their TNM status, which is a more technical item.
Correlating Cancer Staging Systems
A question we are frequently asked is, “How does one correlate these two systems?” In other words, a person might know their stage but not their actual TNM ranking, or vice versa.
It might seem like a straightforward question but it is anything but. The topic is complicated because other factors can influence the determination of the stage. And particular cancers may have their own separate staging system. But as a general rule, to quote the NCI website, most staging systems will include information about:
- Where the tumor is located in the body
- The cell type (such as adenocarcinoma or squamous cell carcinoma)
- The size of the tumor
- Whether cancer has spread to nearby lymph nodes
- Whether cancer has spread to a different part of the body
- Tumor grade refers to how abnormal the cancer cells look and how likely the tumor is to grow and spread. This is a measure of how abnormal a cell looks under the microscope. This is also called differentiation. Cancer cells that are more abnormal-looking tend to grow and spread faster. The grade is usually expressed as a number, with lower numbers (such as G1) used for lower-grade cancers. In low-grade (or well-differentiated) cancers, the cells look fairly normal. These cancers tend to grow more slowly and often have a better outlook. In high-grade (or poorly differentiated) cancers, the cells look more abnormal. High-grade cancers often tend to grow more quickly, so they may need different treatments than low-grade cancers.
Other Staging Factors
Other factors that can affect the stage are the cell type, the tumor location, tumor markers, other lab tests, age, etc. Also, each cancer may have its particular staging system. Here is an example of how in a single case (which is head-and-neck cancers) the TNM system translates into the four-stage system.
Thus, in this particular case, only small tumors with no lymph node involvement and no distant metastasis constitute stage I cancer. A larger primary tumor but still with no lymph nodes or distant metastases is stage II cancer. A still larger tumor with no lymph node or distant metastasis is stage III cancer. But ANY size tumor with lymph node involvement is also stage III. And a very large tumor with any or 1 lymph node involvement is considered stage IV cancer, as is extensive lymph node involvement. ANY degree of distant metastasis with or without any primary tumor or any lymph node involvement is automatically a “stage IVC” tumor, which is the highest stage possible.
But bear in mind that there are over 100 types of cancer, and the staging system for each of them may vary from case to case. Anyone needing further detailed information on this topic should definitely acquire the following classic text:
AJCC Cancer Staging Manual, Eighth Edition